To polish semiconductor wafers (which will be also simply referred to as wafers hereinafter) as typified by silicon wafers, a method for polishing both surfaces at the same time or a method for polishing a single surface is carried out.
FIG. 8 shows a general single-side polishing apparatus 21. The single-side polishing apparatus 21 is constituted of a turntable 23 having a polishing pad 22 attached thereto, a polishing agent supply mechanism 24, a polishing head 25, and the like. With the use of the single-side polishing apparatus 21, a wafer W is held by the polishing head 25, a polishing agent 26 is supplied onto the polishing pad 22 from the polishing agent supply mechanism 24, and the turntable 23 and the polishing head 25 are rotated respectively to slidably contact a surface of the wafer W with the polishing pad 22, thereby polishing one surface of the wafer W.
As a method for holding a wafer by a polishing head, a polishing head using a template is adopted. The polishing head using the template has a relatively simple structure, but it can provide a wafer having a flat shape after polishing by adjusting a depth of a concave portion in which a wafer is housed (which will be also referred to as a pocket depth hereinafter) in accordance with polishing conditions.
FIG. 9 shows a schematic view of a general template. As shown in FIG. 9, a template 1 has an annular member 2 and a backing pad 3 bonded thereto, and an inner peripheral surface of the annular member 2 and an upper surface of the backing pad 3 form a concave portion 4 to house and hold the wafer W. At the time of polishing, the wafer W is housed and held in this concave portion 4. This template 1 is bonded to a polishing head main body by a double sided tape 5 to constitute the polishing head. As a material of the annular member 2, a glass epoxy resin or the like is used.
A pocket depth A′ is obtained by calculating a difference between a height B′ of the annular portion and a height C′ of the backing pad portion. When a thickness of an adhesive layer (not shown) between the annular member 2 and the backing pad 3 is ignored, a thickness of the annular member 2 is the pocket depth A′. Further, a difference between the pocket depth A′ and a thickness F′ of the wafer W is called a wafer protruding amount E′, and an outer peripheral shape of the wafer W can be controlled by managing the wafer protruding amount E′ or the pocket depth A′ under the same polishing conditions.
As extreme examples, a case where an outer periphery of the wafer has a rise shape and a case where the same has a sag shape will now be described with reference to FIG. 10 and FIG. 11.
As shown in FIG. 10, when the thickness of the annular member 2 is larger than the thickness of the wafer W, the wafer protruding amount becomes minus, and the annular member 2 compresses the polishing pad 22. Thus, a retainer effect to reduce a polishing pressure to the outer peripheral portion of the wafer W decreases a polishing amount of the outer peripheral portion of the wafer W, and the outer peripheral portion of the wafer W has the rise shape. On the other hand, as shown in FIG. 11, when the thickness of the annular member 2 is smaller than the thickness of the wafer W, since the wafer protruding amount becomes plus and compression of the polishing pad 22 by the annular member 2 is weakened, the polishing pressure to the outer peripheral portion of the wafer W increases, the outer peripheral portion of the wafer W is abundantly polished, and hence the outer peripheral portion of the wafer W has the sag shape.
In actual polishing, since a magnitude of the retainer effect is determined based on a difference in physical properties between the polishing pad and the backing pad which are elastic bodies, a polishing pressure, or the number of revolutions, the thickness of the annular member is adjusted and an optimum pocket depth is set in accordance with the polishing conditions so that a target outer peripheral shape can be provided.
Each of FIG. 12 and FIG. 13 shows an example of a polishing head using such a template. A polishing head 25a in FIG. 12 is a polishing head which is a type that a template 1 is attached to a surface of a flat and smooth plate 27, and it is called a plate type. Furthermore, considering a material of the plate 27, it is also called a ceramic head or an SiC head.
A polishing head 25b in FIG. 13 is a polishing head which is a type that a template 1 is attached to a surface of a rubber 28, and it is called a rubber head.
Both the polishing heads 25a and 25b have a space using the rubber 28 as a partition wall in the polishing heads 25a and 25b respectively, and a load can be applied at the time of polishing by pressurizing this space with air or the like.
Since each of such polishing heads 25a and 25b does not have a complicated retainer mechanism in the polishing head, management of the polishing head is easy, and a running cost can be suppressed. Moreover, there is no problem of degradation of particles due to dust emission from the retainer mechanism.
However, since an independent retainer mechanism is not provided, a variation in thickness of the annular member of the template or a deviation from a target thickness directly affects a shape of the wafer outer peripheral portion, and hence a problem that quality tends to largely vary occurs as compared with the polishing head having the retainer mechanism.
Thus, for the purpose of suppressing a variation in thickness of the annular member, a method for lapping or polishing the annular member has been suggested. Such a template can suppress an influence of a variation in thickness due to the annular member.
Additionally, to reduce a variation in thickness of the backing pad or a variation in thickness of the template due to a variation in physical properties, there has been also suggested a method for grinding the lapped or polished annular member, engaging a peripheral edge portion of the backing pad with a ground portion, and directly bonding the annular member to a base material such as a double sided tape. Such a template can suppress an influence of a variation in thickness due to the backing pad.
Further, the method for lapping or polishing the annular member is superior in thickness controllability for the annular member, and hence a deviation from a target pocket depth can be also reduced.
FIG. 14 shows a current method for measuring a pocket depth. First, a height B′ of the annular portion and a height C′ of the backing pad portion are measured. Furthermore, a difference between the measured height B′ of the annular portion and the measured height C′ of the backing pad portion is calculated to obtain a pocket depth A′ (A′=B′−C′). In the measurement, a contact type height measuring instrument or a measuring instrument using a laser is employed. In case of using the contact type height measuring instrument, it is often the case that measurement in eight directions is performed by one template and an average value of eight points is used. In case of the non-contact type based on the measuring instrument using the laser, it is often the case that heights of a whole circumference are annularly measured and a calculation is carried out by using their average.
Meanwhile, since the template is fixed to the polishing head by an adhesive or the like and used in this state, it is insufficient as a management method in the measurement before being bonded to such a polishing head as described above, and hence a method for measuring a shape of the backing pad in a state where the template is attached to the polishing head has been suggested (Patent Literature 1).